In recent years, solid electrolytic capacitors have been developed which are formed as follows: a solid electrolytic layer using a conductive polymer as a solid electrolyte and a cathode layer in that order are provided on the dielectric layer formed by oxidizing a surface of an anode body made of a porous valve metal such as aluminum, niobium, tantalum, titanium and magnesium (metals to form an oxidized surface layer).
Such a solid electrolytic capacitor has conductivity that is 10 to 100 times as high as the conductivity of a conventional solid electrolytic capacitor using magnesium dioxide as a solid electrolyte, and is capable of significantly reducing ESR (equivalent series resistance). Accordingly, the solid electrolytic capacitor above is expected to be used for various applications such as absorbing high-frequency noise of compact electronic devices.
In order to form a solid electrolytic layer on a dielectric layer, methods such as a chemical oxidation-polymerization and electrolytic polymerization are generally used.
A chemical oxidation-polymerization method is conducted as follows: a metal, the surface of which is oxidized to form a dielectric layer, is immersed in a solution containing the monomer of a conductive polymer such as 3,4-ethylenedioxythiophene (EDOT), pyrrole and aniline as well as an oxidizer and a dopant (conductive additive) so that the monomer and the oxidizer are directly reacted on the dielectric layer to form a solid electrolytic layer.
On the other hand, an electrolytic polymerization method is conducted as follows: a conductive base layer is formed in advance on the dielectric layer, an electrolyte solution containing the monomer of a conductive polymer and a dopant is applied onto the base layer to form a coated film, and electrical voltage is applied between the coated film and the base layer to form a solid electrolytic layer.
For example, patent publication 1 discloses a method for forming a solid electrolytic layer using a chemical oxidation-polymerization method. Specifically, on a surface of an aluminum electrode on which surface oxidation treatment is conducted, a solution in which EDOT and p-toluenesulfonic acid iron (III) both as an oxidizer and a dopant are dissolved in an organic solvent, is applied to form a polymer-coated film, and then the organic solvent is removed to obtain a solid electrolytic layer.
Also, patent publication 2 discloses the following method: a solid electrolytic layer made of polypyrrole or polyaniline formed by a chemical oxidation-polymerization method is used as a base layer, and another solid electrolytic layer made of the same material is further formed on the base layer using the electrolytic polymerization method.
However, since polymerization reactions are progressed on the dielectric layer by such a chemical oxidation-polymerization method or electrolytic polymerization method, impurities tend to be mixed into the obtained solid electrolytic layer to cause short-circuiting. In addition, the manufacturing process tends to be complex.
Thus, a polymer slurry coating method is proposed to form a solid electrolytic layer without conducting chemical oxidation-polymerization or electrolytic polymerization on the dielectric layer. In such a polymer slurry coating method, a monomer is polymerized in advance to form a polymer (conductive polymer), and a dispersion containing the polymer is applied on the dielectric layer and dried to form a coating film so that a solid electrolytic layer is obtained.
Since, unlike other chemical oxidation-polymerization or electrolytic polymerization methods which carry out polymerization reactions on the dielectric layer, a polymer slurry coating method uses a conductive polymer with completed polymerization by chemically oxidizing a monomer, oxidizer and dopant in advance. Therefore, no polymerization reactions are necessary to be carried out on the dielectric layer, and fewer impurities are mixed into the solid electrolytic layer, making it relatively easy to control the manufacturing process.
However, when a polymer slurry coating method is used, it is difficult to impregnate the dispersion of the conductive polymer deep into the dielectric layer. As a result, a solid electrolytic layer tends to be formed only on the surface layer of the dielectric layer and not inside the fine irregularities (fine pores), thus causing problems such as a lower rate of exhibited capacitance of the obtained solid electrolytic capacitor.
Therefore, another method is proposed to form a solid electrolytic layer by using a conductive polymer soluble in water or organic solvents.
For example, patent publication 3 discloses a method in which a polymer solution obtained by dissolving a certain soluble aniline-based conductive polymer in water or a hydro-organic solvent is applied and dried on a dielectric layer formed by oxidizing the surface of a metal so that a solid electrolytic layer is formed.